A tendon sheath is a layer of connective tissue that surrounds a tendon, primarily in areas where tendons pass through narrow anatomical tunnels, across joints, or over bony prominences. It is composed of two main layers: an outer fibrous layer (fibrous sheath) and an inner synovial layer (synovial sheath). The synovial layer produces synovial fluid, which lubricates the tendon and facilitates smooth, friction-reduced movement during muscle contraction and joint motion.
Structure
The tendon sheath consists of two distinct layers:
- Fibrous layer (fibrous sheath): The outer layer composed of dense collagenous connective tissue that provides structural support and protection.
- Synovial layer (synovial sheath): The inner layer lined by a synovial membrane, which itself has two components — a parietal layer lining the fibrous sheath and a visceral layer covering the tendon surface. Between these layers is a space filled with synovial fluid.
A mesotenon (or mesotenonium), a loose connective tissue peduncle containing blood vessels and nerves, connects the tendon to the surrounding sheath in some locations.
Function
The primary functions of tendon sheaths include:
- Reducing friction: The synovial fluid lubricates the tendon, minimizing friction as it slides against adjacent structures.
- Protection: The sheath prevents the tendon from adhering to surrounding fascia, bones, and ligaments.
- Facilitating smooth movement: By providing a low-friction gliding surface, the sheath allows tendons to move freely, especially in areas requiring sudden direction changes or where tendons pass through fibro-osseous tunnels (e.g., retinacula in the wrist, hand, ankle, and foot).
Location
Tendon sheaths are found throughout the body wherever tendons are subject to increased mechanical friction. Notable locations include:
- Hand and wrist: Flexor and extensor tendon sheaths of the fingers and wrist.
- Foot and ankle: Sheaths surrounding the tendons of the tibialis anterior, extensor hallucis longus, extensor digitorum longus, flexor hallucis longus, flexor digitorum longus, tibialis posterior, and the peroneus longus and brevis muscles.
- Shoulder: The biceps tendon sheath (intertubercular sulcus).
Related Structures
Tendon sheaths are distinct from, but functionally related to, other peritendinous structures:
- Paratenon (peritendon): A loose connective tissue layer surrounding tendons where true synovial sheaths are absent, composed of type I and type III collagen and elastic fibers.
- Bursae: Fluid-filled sacs lined by synovial membrane that cushion tendons and muscles over bony prominences, found near major joints such as the shoulder and knee.
- Retinacula: Fibrous bands that form tunnels through which tendons pass, often in conjunction with tendon sheaths.
Clinical Significance
Several pathological conditions can affect tendon sheaths:
- Tenosynovitis: Inflammation of the tendon sheath, often caused by overuse, injury, or infection. Symptoms include pain, swelling, and difficulty moving the affected joint. Treatment may include rest, ice, anti-inflammatory medications, corticosteroid injections, splinting, or, in cases of infection, antibiotics and surgical intervention.
- De Quervain tenosynovitis: A specific form of tenosynovitis affecting the tendon sheaths of the abductor pollicis longus and extensor pollicis brevis at the radial styloid process.
- Giant-cell tumor of the tendon sheath (GCT-TS): A usually benign, slow-growing tumor arising from the tendon sheath, most commonly in the hand but also occurring in the foot and ankle.
- Fibroma of the tendon sheath: A benign fibrous nodule that may cause swelling or discomfort.
- Infectious tenosynovitis: A serious condition usually resulting from penetrating trauma, requiring prompt surgical intervention and antibiotics.
Terminology
In Latin, the tendon sheath is referred to as vagina tendinis. In anatomical classification systems, it is identified as TA98 A04.8.01.005, TA2 2035, TH H3.03.00.0.00028, and FMA 76715.
